1. Field of the Invention
The present invention relates to a machining device and a production method of an orifice plate and, more particularly, to a production method of an orifice plate for use in an ink-jet printer, wherein the orifice plate is manufactured by micromachining a periodic pattern of a plurality of apertures in a workpiece using coherent light.
2. Description of the Related Art
Machining using coherent light from an excimer laser finds widespread use along with other chemical process and ordinary machining techniques. Since technological advances today present required conditions including materials, optical technology, and production techniques, machining using coherent light is extensively used in the field of micromachining.
FIGS. 13A and 13B are cross-sectional views showing a major portion of the optical system of a machining device the inventors of this invention proposed in Japanese Patent Laid-Open No. 7-230057. As shown, the machining device opens a row of numerous perforations in a workpiece. The optical axis of a convex lens 74 is aligned with the X axis, the direction of the row of perforations of a mask 75 is aligned with the Y axis and the Z axis (perpendicular to the page of the figure) is perpendicular to the plane of the X and Y axes. FIG. 13A is a cross-sectional view of the optical system of the machining device taken along the X-Y plane and FIG. 13B is a cross-sectional view along the X-Z plane.
A laser light source 70 emits coherent light. A prism unit 71 splits an incident laser light into parallel-light beams in three directions, which then enter a shadow mask 72. The shadow mask 72 controls the light amount of three parallel-light beams. A cylindrical lens 73 has a refractive power in the X-Y plane only. A convex lens 74 collects the light beams from the cylindrical lens 73 and guides them to the mask 75. The mask 75 has a pattern according to which a workpiece 78 is going to be machined, and is placed at the focal point of the convex lens 74. The mask 75 has a number of identically sized transparent holes equally spaced along the Y direction with an opaque background.
A projection lens 76 focuses the image of the pattern of the mask 75 on the surface of the workpiece 78 to be machined. The workpiece 78 is light-beam machined according to the pattern image of the mask 75 formed on its surface to be machined. Designated by reference numeral 77 is a diaphragm (entrance pupil) of the projection lens 76.
The operation of the machining device shown in FIGS. 13A and 13B is now discussed in terms of the X-Y plane and the X-Z plane. In the X-Y plane shown in FIG. 13A, the laser light beam emitted from the laser light source 70 is split into three parallel-light beams through the 10 prism unit 71. The three parallel-light beams are restricted in light amount by the shadow mask 72, enter into the cylindrical lens 73, and form three images I.sub.+, I.sub.0, and I_ respectively at the focal point of the lens 73.
The images I.sub.+, I.sub.0, and I_ serve as object points to the convex lens 74, and the light beams from the object points I.sub.+, I.sub.0, and I_ are focused as images I.sub.+ ', I.sub.0 ', and I_ in the diaphragm (entrance pupil) 77 of the projection lens 76 through the convex lens 74. The light beams from the object points I.sub.+, I.sub.0, and I_ are designed to be mutually superimposed at the mask 75. The optical system in the X-Y plane achieves a so-called Koehler illumination, and illuminates uniformly the entire pattern area of the mask 75.
In the X-Z plane shown in FIG. 13B, the prism unit 71 and cylindrical lens 73 simply work as a plane-parallel plate. The light beam from the laser light source 70 is introduced in the form of still parallel light to the convex lens 74, and is focused as a dot on the mask 75.
The optical system in the X-Z plane achieves a so-called critical illumination. The optical system uniformly illuminates the mask pattern area in the X-Y plane through the Koehler illumination, and strongly illuminates a dot pattern in the X-Z plane through the critical illumination. The optical system thus efficiently illuminates the mask 75.
The laser light to which the pattern is imparted is focused on the workpiece 78 through the projection lens 76 in a geometrical-optics manner, and opens holes patterned in the mask at a predetermined magnification there on the workpiece 78.
At the very early stages of industrial applications of a laser subsequent to its development, no optical materials nor optical devices that withstood strong laser beams were available. The optical devices quickly age in time and start presenting a poor transmissivity, resulting in defective products or even get damaged.
Since coherent light such as laser light has a property of a point light source, rather than a diffuse surface illuminance, and a strong monochromaticity, it occasionally presents wave-optics characteristic that are not conventionally encountered. There are times when optical characteristics defects take place which fall outside the conventional defects, such as poor transmissivity of an optical device, attributed to light intensity only.
When a workpiece is made of a material that is easy to machine, the distribution of diffracted light rays is expanded within a lens in design to lower the energy per unit area. However, there is today a growing demand for machining a workpiece that is difficult to machine, such as ceramics and metal. Furthermore, a demand for high-speed machining is also growing. To meet such demands, we are forced to raise the energy per unit and the power factor.
When the power of the light source is heightened or the illuminating pulse rate per unit time is increased to raise the energy per unit area and the power factor, optical materials having a low capacity to withstand lasers suffer variations in refractive index and shrinkage in dimensions. These changes cause variations in an optical path length, leading to optical characteristic defects.
Now, quartz, widely used as an optical material in short wave regions, is considered. A paper by Ohoki in the Institute of Electrical Engineers of Japan Proceedings of 1991, Vol. 1991 No. 3, page S4.15-S4.18 (1991) reported that variations in refractive index of quartz has attributed to variations in an optical path when quartz shrinks in the optical path as a result of an increase of quartz defect and advances of annealing.